(i) Field of the Invention
This invention relates to improvements in the drilling of oil and gas wells in polar regions. More particularly, it relates to improved techniques for effectuating such drilling in the wintertime in the Arctic Ocean and more especially in the shorefast ice area of the Beaufort Sea, although it is feasible for application in other areas where similar conditions exist. Still more particularly, it relates to a novel canopy for a drillship to provide an ice-free zone around a drillship to enable such wintertime drilling.
(ii) Description of the Prior Art
At the present time, drilling in offshore Arctic regions is carried out in the summertime either by the use of drillships anchored at a drill site where the risk of impingement by ice floes is minimal, or through the use of artificial islands. Summertime drilling is feasible for depths of from 60 feet to 2000 feet or more. Artificial islands currently being used in the shallow waste regions of the Beaufort Sea become excessively expensive in water depths of 40 feet or greater. Moreover, it is presently not feasible to drill exploratory wells from floating ice islands in the regions where ice movement is too great (i.e., greater than a few feet).
One of the chief obstacles to overcome in drilling in Arctic regions is the Arctic pack ice. The ice grows to a thickness of approximately 6 feet and is laced with pressure ridges and ice islands which can reach thicknesses of over 100 feet. The pack ice moves at speeds from 0 to 20 or more miles per day with an average movement of approximately 2 miles per day.
If drilling were to take place in waters where there was considerable ice movement, a very solid structure would be required in order to withstand the forces exerted upon it by the ice pack and yet to be able to remain on position in order to drill a well. For exploratory drilling operations, a solid bottom founded structure should be provided which could resist the movement of the ice pack and yet would be mobile enough to be transported from one exploratory drilling site to another.
Drilling below the ice within sea bottom structures has many problems. Firstly, there is the problem of designing structures which could withstand the ice forces which would be exerted against these structures by deep ice keels. In the second place, subsea systems would have problems of buoyancy, life-support systems, power supply, and access for crews and maintenance.
It would, therefore, be desirable to develop another technique that would allow exploratory drilling during winter months. Such system should also be able to extend operational capability into the shorefast ice. The technique discussed hereinabove has the potential for drilling wells more economically than any other method in the shorefast ice regions. Using this technique, the rate of exploration in the Beaufort Sea would be increased by a factor of from two to four times. This technology of using drillships in shorefast ice could be applied to other regions of the Alaskan Arctic. The techniques used for drilling in shorefast ice are a logical step toward developing year-round drilling systems in pack ice regions.
The development of such technology is important since the risks to the environment of a drilling system in the shorefast ice are relatively low. One advantage of operating in shorefast ice is that the ice moves very little throughout the winter. Any oil spilled underneath the ice would be confined to a very small area where it could be removed from the environment.
The initial problem which the present invention proposes to overcome is the maintenance of a substantially ice-free area around the drillship, and in particular, an ice-free area around a drillship operating in shorefast ice zones during winter. Earlier known methods of maintaining an ice-free zone comprised of conventional icebreaking techniques, namely, the use of icebreaking vessels.
One means for solving the initial problem was provided in copending U.S. application Ser. No. 54,997 filed July 5, 1979 now U.S. Pat. No. 4,323,322 for a Warm Air Canopy System for Providing Ice-Free Zone. That invention provided a method for using a passive structural matrix for strategic deployment of a drillship's waste heat to inhibit ice growth around the drillship. The method employed a system of barge-like modules having forced air heating and peripheral facilities, and a series of intermodular roof sections connecting a set of floating barges around the ship. A method was thus disclosed for providing a substantially ice-free zone around a vessel comprising: (a) creating a finite substantially enclosed zone completely around the periphery of the vessel above the waterline, such zone including a peripheral zone approaching the waterline; (b) continuously circulating warm air within the finite substantially enclosed zone; and (c) continuously injecting warm air into such peripheral zone to prevent ice formation within the peripheral zone. A series of such barges was also provided wherein each barge had a hull shaped to ride up on ice sheets, while the entire perimeter of the system was enclosed with a flexible skirt which permitted ice movement underneath while sealing air within the system. Apparatus was also disclosed for providing a substantially ice-free zone around a vessel comprising, in combination with the vessel: (a) a plurality of floating modules disposed around, and connected at one end, to the vessel; (b) a continuous, downwardly depending skirt extending completely around the outer periphery of the plurality of floating modules; and (c) an air/water heat exchanger in a selected plurality of the floating modules.
Another means for solving such initial problem was provided in copending U.S. application Ser. No. 54,661 filed July 5, 1979 for Barge Construction for Warm Air Canopy Ice-Free Zone. That invention provided a barge comprising: (a) a hull including a bow, a stern, a pair of sides, a flat bottom and a deck; (b) a hull stabilization system including a lattice framework provided with solid insulation material; (c) a liquid/gas heat exchange system within the hull having ducts and outlets from the hull to the surface of the water and air recirculation inflow means; (d) a cantilevered section extending forwardly from the box; and (e) a skirt depending from the forward edge of the cantilevered section.
Yet another means for solving such initial problem was provided in copending U.S. application Ser. No. 54,998 filed July 5, 1979 now U.S. Pat. No. 4,270,476 for Novel Skirt Construction. That invention provided a skirt system comprising: (a) a base; (b) a first flexible external skirt depending from the base, the first skirt being provided with a metallic lip depending below the lower edge; (c) a second flexible internal skirt depending from the base adjacent the first skirt and spaced from, and substantially parallel to, the first skirt, the second skirt being slightly shorter than the first skirt; and (d) a compartment providing an air conduit formed by the space between the parallel first skirt and second skirts.